A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to ensure that a target portion of the patterning device is projected on the appropriate position on the substrate during the scanning of the pattern, an accurate positioning of the substrate relative to the patterning device is desirable. Accurate positioning of the patterning device and substrate is, is in general, achieved by the application of a plurality of electromagnetic actuators and motors. At the same time, it is desirable for a lithographic apparatus to realize a high throughput, i.e. the number of wafers processed over a given time should be as high as possible. In order to achieve a high throughput, a high scanning velocity is preferred. Achieving a high scanning velocity requires the application of high acceleration and deceleration forces. Known positioning devices of lithographic apparatuses include an actuator assembly to provide an accurate positioning of the substrate table. Such an actuator assembly is in general arranged below the substrate table. It has been observed that such an arrangement of an actuator assembly may result in unwanted deformations or displacements of the substrate table during the operation of the positioning device thereby adversely affecting the accurate positioning of the substrate table. In order to avoid such unwanted deformations or displacements, complex control strategies or additional actuators may be required.